Corona charging apparatus



Feb. 20, 1968 L. F. FRANK' 3,370,212

CORONA CHARG I NG APPARATUS Filed Aug. 19, 1965 LEE F F RANK INVENTOR.

BY flaw/M ATTORNEYS 3,370,212 Patented Feb. 20, 1968 United StatesPatent Ofiice 3,370,212 CORONA CHARGING APPARATUS Lee F. Frank,Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., acorporation of New Jersey Filed Aug. 19, 1965, Ser. No. 480,980 Claims.(Cl. 317--262) ABSTRACT OF THE DISCLOSURE A unilaterally conductingcircuit for rectifying an AC. corona current as a pulsed D.C. coronacurrent is produced by modulating an alternating current coronadischarge by interposing between the corona source and the grid plate ofan electrographic plate a conductive grid connected to ground through arectifier.

This invention relates primarily to electrograph'y and in a preferredembodiment thereof to a direct current corona source useful in chargingelectrographic plates.

It is usual in electrography to uniformly charge an in sulating plate(which becomes conductive upon activation), to imagewise activate theplate to produce an electrostatic image thereon and then to develop theelectrosatic image. The imagewise activation can be by imagewiseexposure to light as in xerography or by imagewise heating as inxerothermography, for example.

Conventional electrographic charging sources employ an A.C. voltagesource connected to a corona discharge electrode through a series ofrectifiers and capacitors to obtain a high DC. potential at theelectrode. The use of such high voltage components to rectify the AC.current and to properly handle the total current demands of the systemadd to the expense of the electrographic device (whichmay be, forexample, a document copier). One

corona charging source described in the literature employs, between thecorona discharge electrode (in this case connected directly to an AC.potential source) and 'the plate to be charged, a control electrodehaving a DC. voltage impressed thereon to regulate the flow of ions.

It is an object of the present invention to provide a DC. corona sourcewhich is both simple and inexpensive.

It is a further object of the invention to provide an electrographiccorona charging apparatus in which only low voltage rectifiers arerequired and in which only a fraction of the total current drawn by thepower supply needs to be rectified.

Still another object of the invention is to provide a simple andeconomical D.C. corona source which additionally provides for thestabilization of the spatial distribution of the corona current.

It is a still further object of the invention to provide a simple andeconomical, pulsed D.C. corona source which additionally provides forthe control of the phase and amplitude of the pulsed D.C. coronacurrent.

These objects are accomplished by the present invention as follows. Apulsed D.C. corona discharge is produced, according to the presentinvention, by modulating an alternating current corona discharge byinterposing, between the corona source and the ground plate of theelectrographic plate or sheet, a conductive grid connected to groundthrough a rectifier. Only about 10% of the current drawn from thecurrent source passes through the rectifier; thus, the rectifier mustcontrol only about 1% or less of the power that would be rectified ifthe rectifier were put in series with the alternating current source asis done in the above-described prior art. The above-described prior artwhich employs an AC. potential source connected directly to the coronaelectrode and a DC. voltage impressed on a control grid requires, inpractice, more components than the simple rectifier of the subjectinvention, i.e., it requires a transformer, a capacitor, and an extraelectrical lead to the corona electrode. Further, the rectifier of thesubject invention does not cause a constant potential but rather apulsating potential on the grid,-

varying between a couple of hundred volts when corona is beingtransmitted and zero volts when it is not. In the subject invention, thecontrol voltage is derived from the current to be controlled instead offrom an external power source. Important advantages of the presentinvention are (1) only low-power rectifiers are required, (2) the gridacts to stabilize the spatial distribution of the corona current and (3)the phase and amplitude of the pulsed D.C. corona can be controlled, Inone embodiment of the present invention, instead of connecting the gridto ground through a rectifier, a rectifying coating, such as anunsensitized zinc oxide in resin binder layer, is coated on a groundedgrid. In this embodiment, of course, only negative ions can betransmitted. Various types of rectifying coatings, however, may beemployed to pro- I vide the transmission of the desired polarity ofcurrent.

These and other objects of the present invention will be more fullyunderstood by reference to the following detailed description when readin connection with the accompanying drawings in which:

FIG. 1 is a schematic illustration of one embodiment of the presentinvention, and

FIG. 2 is a schematic illustration of another embodiment of the presentinvention.

FIG. 1 shows a high-voltage AC. power supply 2 connected through switch4 to a corona discharge electrode 6. The other terminal of the powersupply 2 is connected to a conductive backing 8 of a Xerographic sheetor plate 10 having a photoconductive coating 12 on the conductivebacking 8. According to the present invention, a conductive grid 14 ispositioned between the corona discharge electrode 6 and the plate 10.The grid 14 is connected to ground through a rectifier 18. When thecorona discharge electrode 6 is energized by closing the switch 4, thegrid 14 operates to rectify the alternating current which flows from theelectrode 6 to the plate 10 whereby the photoconductive layer 12 isprovided with a uniform electrostatic charge of a given polarity.Various methods are known for uniformly corona charging aphotoconductive layer and anyof these methods, for example the use ofcontrol grids or shields or the relative movement of the coronadischarge electrode and the plate 10, may be used in the presentinvention.

. During the positive half of the above described alternating current,the ions or charges incident on the grid 14 pass through the rectifier18 to ground. When the corona current is negative, the rectifier willnot conduct the current, and the grid 14 is quickly charged up to thespace potential appropriate to its position and then immediately ceasesto intercept current whereby the remaining current incident on the grid14 passes through the grid 14 to charge the photoconductive layer 12.

Only pulsating D.C. corona current is transmitted through the grid 14for most of the frequencies involved in alternating current. A groundedmetal shield 20 partially surrounding the corona discharge electrode 6may be used to stabilize the corona further at relatively high coronalevels. The total potential across the rectifier 18 is limited to about200300 volts. Usually only about 10% of the current drawn from thecurrent source passes through the rectifier 18. It is easily seen thatthe rectifier 18 must control only about 1% or less of the power thatwould be required if it were put in series with the power supply 2. Thissystem works equally well with the rectifier 18 reversed and producesthe opposite charged current incident on the photoconductive layer 12.If it is found that the rectifier 18 does not completely 3 rectify theAC. corona current, a bias potential 22 which can be connected in thecircuit by means of a switch 24 will guarantee that the A.C. coronacurrent will be completely rectified.

FIG. 2 shows another embodiment of the invention, with like numeralsindicating like elements. The only difference between theembodimentshown in FIG. 2 and that shown in FIG. 1 is the use of arectifying coating 26 on the grounded grid 14- instead of the use of therectifier 18 between the grid 14 and ground. This coating may be, forexample, a layer of unsensitized zinc oxide in resin binder. Variousrectifying coatings may be en1- ployed and the polarity of the currenttransmitted to the photoconductive layer 12 is dependent on the powercharacteristics of the material embodied in the coating. In the case ofunsensitized zinc in resin, only negative current is transmitted. A biaspotential 22 can be connected to the grid 14 by means of a switch 24 ifdesired.

As stated in the first paragraph of the specification, the presentinvention relates only primarily to electrography. The corona source ofthe present invention can be enclosed in a box with the grid 14- as theonly opening to the air and this assembly can be used as a source of airions of a given polarity, such as are used, for example, in those airconditioning units which have a dust trap. Various other uses of thesubject invention will be obvious to those skilled in the art. Othermodifications of the invention include the use of a silicon controlledrectifier to synchronize the partial phase of the pulse with a morecomplex circuit or to produce a pulse-width, modulated pattern. Further,a piezoeleoctric voltage source may be substituted for the customaryhigh-voltage transformer in order to provide an even more economicalsystem. A piezoelectric source of high-voltage may also be used foroperation of the subject system away from sources of electrical power.Piezoelectric voltage sources are current alternating for continuoususe.

The present invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof, but itwill be understood that variations and modifications can be effectedwithin the spirit and scope of the invention as described hereinbeforeand as defined in the appended claims.

I claim:

1. Apparatus for producing a DC. corona current comprising:

means for producing an AC. corona current, and

means for rectifying said current to produce a pulsed,

D.C. corona current comprising a grounded conductive grid having arectifier in series circuit therewith interposed in said A.C. coronacurrent, said grounded grid and rectifier forming a unilaterallyconducting circuit for rectifying said A.C. corona current.

2. The apparatus according to claim 1 in which said rectifier comprisesa coating of rectifying material on said grid.

3. The apparatus according to claim 1 in which said grid is connected toground through said rectifier.

4-. Apparatus for charging a surface comprising:

means for directing an AC, corona current toward said surface, and

means for rectifying said A.C. corona current to produce a pulsed, D.C.corona current which impinges on said surface comprising a groundedconductive grid having a rectifier in series circuit therewithinterposed in said A.C. current, said grounded grid and rectifierforming a unilaterally conducing circuit for rectifying said A.C. coronacurrent.

5. The apparatus according to claim 4 in which said rectifier comprisesa coating of rectifying material on said grid.

6. Apparatus according to claim 5 in which said grid is connected toground through a rectifier.

7. An electrographic apparatus for charging the surface of anelectrographic sheet having a grounded conductive backing comprising:

means for supporting said sheet in a charging plane,

a corona discharge electrode spaced from said surface,

means for applying an electric field between said electrode and saidbacking,

means for applying a corona generating A.C. potential to said electrode,and

a ground conductive grid positioned between said electrode and saidsurface and having a rectifier in series circuit therewith, saidgrounded grid and rectifier forming a unilaterally conducting circuitfor rectifying said A.C. corona current to produce a pulsed, D.C. coronacurrent.

8. Apparatus for producing a DC corona current comprising:

means for producing an AC. corona current, and

means for rectifying said current to produce a pulsed,

D.C. corona current comprising a conductive grid interposed in said A.C.corona current, said grid having a rectifier and a source of biaspotential in series circuit therewith, said grid, rectifier, and sourceforming a unilaterally conducting circuit for rectifying said A.C.corona current.

9. The apparatus according to claim 8 in which said rectifier comprisesa coating of rectifying material on said grid.

10. The apparatus according to claim 8 in which said grid is connectedto ground through said rectifier and said source of bias potential.

References Cited UNITED STATES PATENTS 3,068,356 12/1962 Codichini25049.5 2,982,647 5/1961 Carlson et al. 250-49.5 X 2,890,343 6/1959Bolton 250-495 2,879,395 3/1959 Walkup 317-262 X 2,778,946 1/1957 Mayo25049.5

MILTON O. HIRSHFIELD, Primary Examiner,

I. A. SILVERMAN, Assistant Examiner,

